1. Field of the Invention
The present invention relates to a linear-modulation type radio transmitter which is capable of adaptively controlling mean transmission power.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a conventional radio transmitter. In the same drawing, there are shown an FM modulator 1 for performing frequency modulation (hereinafter called "FM"), a variable attenuator 2 for applying predetermined attenuation to an FM wave from the FM modulator 1, a power amplifier 3 for performing power amplification of the FM wave attenuated by the variable attenuator 2, a transmission power amplifier 4 consisting of the variable attenuator 2 and the power amplifier 3, a directional coupler 5 connected to the transmission power amplifier 4 and for detecting a progressive wave from outputs delivered from the transmission power amplifier 4, and a transmit antenna 6 connected to the directional coupler 5 and for radiating in space an FM wave subjected to power amplification of the transmission power amplifier 4.
In the same drawing, there are further shown a level detector 7 for detecting the level of the progressive wave detected by the directional coupler 5 and for producing a detection voltage corresponding to the voltage level of the progressive wave, a differential amplifier 8 for amplifying the difference in voltage, i.e., an error voltage between the detection voltage from the level detector 7 and a reference voltage, a reference voltage selecting circuit 9 for selectively supplying reference voltages of V.sub.1 -V.sub.n, which have been set in advance in association with a plurality of power outputs to be transmitted from a predetermined antenna, to the differential amplifier 8, a loop filter 10 serving to determine the frequency-response characteristics of the loop, and an error amplifier 11 consisting of the differential amplifier 8 and the reference voltage selecting circuit 9.
A description will now be made of the operation of the radio transmitter referred to above. After an FM wave generated from the FM modulator 1 has been subjected to predetermined attenuation by the variable attenuator 2, maximum power is delivered to the same by the power amplifier 3. The FM wave, which has been subjected to power amplification, is fed via the directional coupler 5 to the transmit antenna 6 from which it is radiated in space.
On the other hand, the directional coupler 5 detects a progressive wave from the FM wave subjected to power amplification and then applies the same to the level detector 7. Then, the level detector 7 converts the voltage level of the progressive wave applied thereto into a detection voltage corresponding to the voltage level thereof, and thereafter applies the same to one of the input terminals of the differential amplifier 8. A reference voltage, which has been set at the time of input of the detection voltage and corresponds to power to be output from the transmit antenna, i.e., a reference voltage V.sub.1, is selected by the reference voltage selecting circuit 9 in the illustrative example and applied to the other input terminal of differential amplifier 8. Then, the differential amplifier 8 serves to amplify the difference in voltage, i.e., an error voltage between the detection voltage from the level detector 7 and the reference voltage V.sub.1 from the reference voltage selecting circuit 9, and then feeds back the amplified voltage to the variable attenuator 2 in the transmission power amplifier 4 through the loop filter 10.
The variable attenuator 2 controls the level of attenuation based on signals from the loop filter 10. As a result, the sum of the gain corresponding to the level of attenuation by the variable attenuator 2 and the gain of the power amplifier 3 is adjusted such that the error voltage between the detection voltage from the level detector 7 and the reference voltage V.sub.1 from the reference voltage selecting circuit 9 becomes smaller. Thus, the transmission power of the FM wave, which is radiated in space from the transmit antenna 6, is automatically controlled to a predetermined value which has been set in advance, without being affected by variations in the supply voltage, changes in temperature or the like.
FIG. 2 is a circuit diagram showing one embodiment of a transmission power amplifier 4 which is constructed of field effect transistors or the like. It is found from its circuit diagram that a modulated carrier wave applied to the input terminal is amplified with an amplification factor depending on a power amplification control signal applied to a control input terminal and the amplified carrier wave is output as an amplified signal from the output terminal.
As the conventional radio transmitter has been constructed as described above, the transmission power can be controlled automatically where the envelope of the modulated carrier wave does not rely on the modulating signal and is constant, as in the case where frequency modulation is employed. Where the linear modulation is of such a type that the envelope of the modulated carrier wave varies according to the modulating signal employed, the envelope component of the modulated carrier wave is suppressed by the feedback control, thereby causing a problem in that information borne by the modulated carrier wave itself is attenuated or destroyed It is desired to prevent variations in the envelope component by increasing the time constant of the feedback loop. However, by increasing the time constant, the response time for power ramping increases; thus this method cannot be applied to intermittent burst transmission.